Frequency modulation receiver tuning indicator



INVENTOR ATTORNEY June 16, 1942- E. w. HEROLD FREQUENCY MODULATIONRECEIVER TUNING INDICATOR Filed March 25, 1941 .2 Sheets-Sheet 1 June16, 1942. E. w. HEROLD FREQUENCY MODULATION RECEIVER TUNING INDICATORFiled March 25, 1941 2 Sheets-Sheet 2 INVENTOR Zi'imznl Wfierplzi' BY kATTORNEY Patented June 16, 1942 uurro STTES PATENT Price FREQUENCYMODULATION RECEIVER TUNING INDICATOR Edward W. Herold, Verona, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication March 25, 1941, Serial No. 385,083

12 Claims.

converted to a. wave which is both amplitude and frequency modulated,the converted wave then being rectified. In other words, the frequencymodulations on the transmitted carrier wave, which appear as frequencydeviations of the center frequency, are converted to an audio frequencysignal corresponding to the original mod- ,ulating signal, the audiofrequency signal being amplified and converted to sound by a loudspeaker. As is well known, a discriminator network is used which usuallyembodies a pair of cascaded reactively coupled resonant circuits eachtunedto the center frequency of the applied frequency modulated waves.side of the first tuned circuit is capacity coupled to the midpoint ofthe second tuned circuit. Opposed rectifiers are used in such a mannerthat when the applied FM waves have a center frequency equal to thecenter frequency of the cascaded resonant circuits, then'acrossthecommon load of the opposed rectifiers zero rectified voltage isproduced. When the center frequency of the applied waves departs fromthe center frequency of the discriminator circuits, then dinot currentvoltage is produced of a polarity and magnitude dependent upon thedirection and extent of the frequency deviation from the cen-.

ter frequency of the discriminator circuits.

In the past there has been utilized a visual tuning indicator forenabling the set operator to tune the receiver 50 that the centerfrequency of the frequency modulated waves would beexactly equal tothe-center frequency of the tuned circuits of the discriminator network;The

. point of accurate tuning occurs at the center of the discriminatorcharacteristic, and, as is well known, the mean carrier frequency of theapplied waves must be held at this point for best signal to noise ratio.However, when using an indicator of the well known electronic shadowtype it has been found that a false reading will be secured when signalsare not present, or when The high potential the signal is far off tune.Since the shadow type of indicator tube depends upon a minimum angle ofshadow to indicate accurate tuning, the production of such minimumangle, when weak or no FM waves are received, is confusing andmisleading to the person tuning the receiver.

Accordingly, it may be stated that it is oneof the main objects of thisinvention to eliminate the aforesaid defect, and to have the shadowangle attain its minimum value solely when the center frequency of thereceived FM waves exactly equal to the center frequency of thediscriminator tuned circuits, this being the condition for accuratetuning.

Another important object of this invention may be stated to reside inthe provision of a visual current indicator of the electronic shadowtype which has a predetermined shadow angle to'indicate accuracy tuningwhen used with. an FM receiver, and means being employed, which isresponsive to the magnitude of the carrier amplitude, for exercising acontrol over the indicator tube in a sense such that the said desiredshadow angle is secured solely for accurate tuning of the receiver.

Further objects of myinvention are to improve tuning indicator meanswhich are applicable equally to receivers of both frequency andamplitude modulated carrier waves, and to provide I itself, however, asto both its organization and method .of operation will best beunderstood by reference to the following description taken in connectionwith the drawings in' which I have indicated diagrammatically severalcircuit organizations whereby my invention may be carried into effect.

In the drawings: 1 shows a receiving invention,

Fig. 1a graphically showsthe discriminator characteristics,

Fig. 2 illustrates. graphically the advantage .of the present inventionover the prior art,

Fig. 3 shows a modification of the invention.

Fig. 4; shows a tube construction that may be used in conjunction withthe indicator circuit, Fig. 5 shows a modified tube construction.Referring to the accompanying drawings, wherein like referencecharacters in the diflferent system' embodying the figures indicatesimilar circuit elements, the cir-- cuit arrangement in Fig. 1represents that portion of an FM receiver of the superheterodyne typefollowing the last intermediate frequency (I. F.) amplifier. It is notbelieved necessary to show the various circuits up to the limiter stage,since they are well known to those skilled in the art. In general, anydesired type of signal collector may be used, and particularly one, suchas a dipole, which is able to collect desired FM waves in the 42 to 50megacycle (me) band. Following amplification at ultra-high frequency,the desired FM wave is converted in any desired type of converternetwork to an FM wave whose center frequency is at an I. F. value. Forexample, such I. F. value may be 4.2 me, or, in general, any valuechosen from a range of 2 to mo.

Subsequent to amplification at I. F., the FM waves are applied to thetuned input circuit l of the limiter tube 2. The limiter tube may beconstructed in the manner of a saturated amplifier, and usually isprovided with a resistor 3 in its grid circuit shunted by an I. E.by-pass condenser The network 3-4 is chosen, in addition to theconstants of the plate circuit, so that the limiter functions to producein its resonant output circuit 5 waves which have substantially noamplitude variation. In other words, the limiter stage has aninput-output characteristic which rises up to a predetermined inputamplitude, and then is substantially fiat thereafter.

The output circuit 5 is tuned to the center frequency fc of the FMwaves, and, of course, fc will be of the LF. value, it being pointed outthat the input circuit l is also tuned to the value Jc. The circuit 5 isconnected to a mid-tap on the coil of the following resonant circuit 5through a coupling capacity I. The circuits 5 and 6 are alsomagnetically coupled. It is not believed necessary to describe in detailthe manner in which the discriminator network '5@ functions, since thathas been disclosed and claimed by S. W. Seeley in U. S. Patent 2,121,103granted June 21, 1938. It is sufiicient for the purpose of thisapplication to point out that the discriminator has a characteristic asdepicted by the full line in Fig. 1a. The characteristic is an inclined,S-shaped curve which is a result of the currents flowing inthe doublediode rectifier tube 8. The latter may be of the 6H6 type. This tube hasa pair of diodes disposed therein, and the anodes of the two diodes areconnected to opposite sides of the resonant circuit 6.. The cathodes ofthe diodes are connected to each other through a pair of series relatedload resistors 9 and Ill, the junction of the resistors being connectedto the midpoint on the coil of resonant cir cuit 6 through a radiofrequency choke coil H. The condenser 12 is shunted across resistiveload 9-H) to by-pass I. F. currents, and audio volttage developed acrossresistive path 9-60 is transmitted through condenser I 3 to the audiofrequency amplifier network. I

As shown by the characteristic of Fig. 1a when the applied FM waves havethe center'frequency located at is, which is the predetermined frequencyof the discriminator circuits 5 and 6, then the rectified voltageproduced across 9--Hl will be an accurate reproduction of the-modulationsignal on the carrier wave. This occurs because at one side of theresonance frequency fc one of the diodes is impressed with a highercarrier voltage than the other diode. At the other side of resonance thelatter diode has a higher voltage than the first diode.

= rod 30 and target 21. f

comes less positive than the target 21 it deflects This behavior is dueto the changed phase difference between the primary and secondaryvoltages existing across circuits 5 and 6 as the carrier frequency isdeviated with respect to ,fc. At exact resonance with fc each diode hasequal carrier voltage applied thereto. As shown by the dottedcharacteristic curves in Fig. 1a, first one diode then the otherreceives maximum voltage as the frequency of the carrier is variedrelative to fc. At the exact fc value the two diode currents are equal,and the voltages produced across t and in exactly cancel. The periodicvariations of the incoming frequency, as in frequency modulationreception, gives rise to a periodically varying output across 9-40 whichcorresponds to the original modulation. The modulation voltage developedacross path 9-40 is, of course, the rectified voltage corresponding tothe instantaneous deviations from in.

However, the direct current voltage produced across 9-Hl is utilized forvisual tuning indica tion. The direct current voltage represents theaverage, or mean, departure of the center frequency of the applied FMwaves from the frequency of the discriminator circuits 5 and 6. Such adeparture produces a direct current voltage across 9 and ill whosepolarity and magnitude depends upon the direction and extentrespectively of departure of the center frequency from 5%.

In the past the direct current voltage so produced has been utilized fortuning indication by connecting in shunt with resistors 9 and it asecond pair of resistors E i and I 5 whose junction is connected to asource of negative bias voltage. An audio by-pass condenser is connectedbetween the lower end of resistor l5 and ground. A tube housing a pairof triodes acts as a direct current voltage amplifier of the directcurrent voltages developed across resistors It and I5. The

cathodes of the triodes are connectedto ground.

The grid l8 of one triode is connected through filter resistor E9 to theupper end of resistor M. The grid 20 is connected through filterresistor 29 to the lower end of resistor l5. Each of the grids of thepair of triodes is connected to ground through capacitance to by-passaudio frequency voltage components.

The plates 22 and 23 of tube I! are connected in parallel to thepositive, terminal of a source of direct current through a resistor 24.The electronic shadow indicator tube is designated by numeral 25, andcomprises an electron emission element 26 which is established at groundpotential. The target, whose inner face is coated with fluorescentmaterial, is designated by numeral 21, and it is connected to theaforementioned positive terminal of the direct current source whichenergizes the plates of tube H. The shadow control rod is denoted bynumeral 30,

and has a direct current voltage connection to the plate end of resistor2 1.

As is well known to those skilled in the art, the rod 30 deflectselectrons from the adjacent portion of the fluorescent target to anextent dependent upon the potential difference between That is, if therod 30 beelectrons, and, therefore, produces a shadow or dark area onthe target portion in front of it. The shadow angle becomeswiderdepending upon 1 the extent to which rod 30 becomes less positive.

When the plate current of both of the triode sections of tube I! are cutoff by a sufiiciently negative bias applied to each of grids l8 and 20,

the shadow produced on target 21 by rod 30 is of minimum angle. In thatcase the voltage drop across resistor 24 is substantially zero.

In normal operationgrids l8 and 20 receive an initial bias from theaforementioned negative bias source through resistors I4 and I5respectively. In other words, each of grids I8 and is normally biased soas to prevent the flow of plate current through resistor 24. When thereceiver is now tuned so that the applied FM waves have a centerfrequency exactly equal to fc, each of the diodes of tube 8 have equalaverage currents so that the total'directcurrent voltage drop across 9and I0 is zero. The grids l8 and 2|! areftherefore, at the samepotential, and minimum shadow angle on theindicator is produced.

If, now, the receiver tuning is changed so that the mean carrierfrequency of the applied FM waves is no longer of the value ,fc, theaverage current of one diode of tube 8 exceeds that of the other,.and anet direct current voltage exists across resistors 9 and Ill in series.This net voltage results in the grid of one triode section of tubereceiving a more negative bias, and the grid of the other triode sectionthe less negative bias. The latter triode will thendraw plate current,and the shadow angle on the target will become greater. correct tuningposition (f0) the shadow angle will increase. It will, now, be notedthat when no signal is present, or when the signal is very far offtune,-the average current of each diode of tube 8 is very small andsubstantially equal to that of the other diode.

The shadow angle is, therefore, a minimum for this condition giving thesame indication as for a correctly-tuned signal. This prior operation ofthe tuning indicator is more clearly brought out by means of thecharacteristic curve shown in Fig. 2. In this figure there is plottedgraphically the angle of the shadow on the target of indicator tube as afunction of frequency to which the receiver 'is tuned, The dotted linecurve labeled Prior art shows that as the actual carrier frequencyisapproached from the left, the angle of the shadow increases from zero,passes through a maximum, then decreases through zero, andth'en'increases once more and passes through a positive maximum andfinally decreases to zero. The point of accurate tuning is marked by thetrough located at the center. It is seen that when no signal is present,that is, for example, when the receiver is very far offtune, the shadowis of a minimum angle. This action is undesirable because it isconfusing to the person who is tuning the receiver. The defect, or falsereading, is caused by the fact that the grids of the two triode sectionsof tube I1 receive a fixed bias independent of the incoming signal.

According to my invention this false indication is eliminated byproviding a third electron discharge tube, illustrated in Fig. 1 as atriode 3|. The plate 32 is connected to the ray control electrode alongwith the plates 2-2 and 23 of the other triode sections. connected to asource of negative direct current voltage which is derived from, orcontrolled by, the incoming signal itself. For example, the grid 33 isshown connected through a lead 34 which may also be the automatic volumecontrol (AVC) lead. The, limiter resistor 3 develops direct currentvoltage, dependent upon carrier amplitude, which is sometimes. used inFM receivers as a source of AVG bias to control the On either side ofthe- The grid 33 of tube 3| is signal grids of prior ultra-highfrequency, and I. F. amplifiers. The network 35 filters out pulsatingvoltage components from theAVC bias, and the filtered voltage is appliedover lead 34 to grid 33. Lead 34 is connected to the grid end ofresistor 3.

The operation of the indicator circuits differs now from prior artoperation in the manner shown by the full line curve in Fig. 2 labeledWith present circuit. When no signal is present the tube 3| has nonegative bias on its grid 33, and its plate current, therefore, flowsthrough resistor 24 thereby keeping-the shadow angle on either side off0 fairly wide. Since the oathode of tube 3| is grounded, the platecurrent of tube 3| necessarily flows through resistor 24 and developsayoltage which establishes control rod 30 at a negative potential withrespect to target 21. However, when the receiver is tuned close to thedesired value fc, there is applied to grid 33 sufficient negative bias,from the limiter or AVC circuit, to cut off the plate current of tube3|. From this point the tuning is correctly done as before, but thefalse reading of the shadow angle is completely avoided, this beingclearly shown by the full line curve in Fig. 2. Such a characteristic issubstantially the same as is obtained in the usual amplitude'modulatedcarrier wave receiver using a variable shadow indicator tube of the typeshown in Fig. 1.

It will be noted that in receivers intended for the reception of eitheramplitude or frequency modulation, my invention continues to beoperative without change in connection. In such receivers it isnecessary, of course, to obtain the audio frequency signal from thecarrier at some other point in the system when receiving amplitudemodulated carrier waves. In other words, the FM discriminator cannot beused to receive amplitude modulated carrier waves. Alternatively,however, a switch in the connections of the discriminator may be madecorrectly to receive either type of signal.

It should be clear that the bias for the grid of tube 33 may be obtainedby-a variety of means which are apparent to those skilled in the art.The limiter grid current provides an automatic grid bias which isroughly proportional to the signal carrier, and, hence, provides a readyand convenient means to prevent the false minimizing of the shadowanglewhen no signals are received.

In Fig. 3 is shown a modified arrangement which shows the possibility ofobtaining the control bias for tube 3| from the discriminator itself. Inthis circuit, which is exactly similar to that shown in Fig. 1 exceptfor the details to be' hereinafter explained, the resistor 40 hasdeveloped across it a direct current voltage which depends on the sum ofthe two diode currents of tube 8. The resistor 40 is arranged in serieswith' the choke .coil II, and is in the common path of both diodes. Thevoltage developed across re-' sistor 40 is, therefore, dependent uponthe signal carrier amplitude, but does not vary with tuning when thelatter is close to the correct adjustment. The grid 33 is connectedthrough filsistors M and [5. It will now be seen that in the case of thecircuit of Fig. 3, and in the absence of received signals, the platecurrent of tube.3i flows through resistor 24' and provides the increasedextremities of the full line curve of Fig. 2. As signals are receivedthe carrier amplitude results in an increased voltage, being producedacross resistor 40. This ultimately cuts oi? the plate current of tube3i, and then the indicator tube is responsive solely to the directcurrent voltage developed across resistors 9 and- Ill.

Since there are tubes commercially available which include a triode anda tuning indicator section of the type herein considered in a singleenvelope, it will be apparent that the most economical application ofthe circuits of Figs. 1 and 2 will make use of such a tube. In this casethe triode M will be in the same envelope as the tuning indicator tube.The cathode of tube 3i may be a portion of the cathode 26 of theray-control tube.

From the standpoint of economy, it may be desirable to combine thetriode sections of tube I! and the triode section of tube 3|] in onetube envelope using a common cathode and a common plate as shown in Fig.4. In this latter figure the tube 50 has a cathode 5! which wouldprovide the electrons flowing to'the plate 52. In axial alignmentbetween the common cathode 5i and common plate 52 would be arranged thecontrol grid Hi, the control grid 20 and the control grid 33. It will beseen that this tube can be used in the circuit of Fig. 1, because thethree plates of tube l7 and tube 35 are connected to a common point onresistor 2 3, and the three cathodes are all at ground potential.

In Fig. 5 there is shown another tube construction which may be used.Commercially available tubes can be had which include a single triodeand a variable shadow indicator section. Such a tube may be made toinclude all three control grids necessary to my invention. Hence, thetube Bil of Fig. 5 may have a cathode section 5| emitting electrons to acommon plate 52', the control grids I8, 20 and 33 being arranged inaxial alignment between the cathode 5i and plate 52.. The cathodesection 5! may have a second emission section 26 corresponding to thecathode 26 of Fig. 1, a target section 21' and a ray control rod 30'. Itis believed that it will be obvious to those skilled in the art how thevarious electrodes of Fig. 5 are to be connected to the differentportions of the circuit of Fig. 1 or Fig. 3.

While I have indicated and described several systems for carrying myinvention into effect, it

will be apparent to one skilled in the art that my' invention is by nomeans limited to the particular organizations shown and described, butthat many modifications may be made without departing from the scope ofmy invention, as set forth in-the appended claims. The expression timingmodulated carrier wave used in the claims is to be understood asgenerically covering frequency, or phase, modulated carrier waves.

What I claim is:

1. In combination with a timing modulated carrier wave detector havingan input circuit upon which are impressed timing modulated carrierwaves, said detector having an output circuit ,across which is developeddirect current voltage whose polarity and magnitude depend uponfrequency departure of the center frequency ofthe applied modulatedwaves from an assignedcenter frequency value of said detector tubeprovided with a shadow control electrode, means for varying thepotential of said shadow control electrode in response to said detectordirect current voltage output whereby the shadow angle of the indicatortube indicates the said frequency departure, means for controlling thepotential of said shadow control electrode in the absence of modulatedwaves of a predetermined amplitude at said input circuit for maintainingthe shadow of said indicator tube at maximum width, and additional meansresponsive to carrier amplitude increase for rendering said lastnamedmeans ineffective.

2. In combination with a detector having an input circuit upon which areimpressed timing modulated carrier waves, said detector having an outputcircuit across which is developed direct current voltage whose polarityand magnitude depend upon the frequency departure of the centerfrequency of the applied modulated waves from an assigned centerfrequency value of said detector input circuit, an electronic shadowindicator tube provided with a ray control electrode, means for varyingthe potential of said control electrode in response to said detectordirect current voltage output whereby the shadow angle of the indicatortube indicates the said frequency departure, means for controlling thepotential of said control electrode in the absence of modulated waves atsaid input circuit for maintaining the shadow of said indicator tube atmaximum width, additional means, responsive to carrier amplitudeincrease, for rendering said last named means ineffective, said lastnamed means consisting of an electron discharge tube, means for derivinga second direct current voltage from received modulated waves, and meansfor applying said second direct current voltage to said last tube in asense to prevent plate current fiow thereof.

3. In a frequency modulation receiver provided with a detection networkof the type comprising a pair of rectifiers having a common inputcircuit tuned to a predetermined center frequency, an output circuitacross which is developed a direct current voltage whose polarity andmagnitude are a function of the frequency departure of applied modulatedwaves from said center frequency, a direct current voltage amplifier foramplifying said direct current voltage, an indicate; tube provided withan emission electrode, a fluorescent target, and an electron ray controlelectrode, means for applying the direct current voltage output of saidamplifier to said ray control electrode thereby to control the width ofangle of the shadow produced on the target, and means responsive to adecrease of modulated carrier amplitude below a desired value formaintaining the shadow on said target at a relatively input circuit, anelectronic shadow indicator wide angle.

4. In a receiver for frequency-modulated waves including a selectiveamplifier and a balanced detector whose rectified output issubstantially proportional, over the normal operating range, to thedeparture in frequency of a selectively amplified incoming signal fromits mean value, a tuning indicator means of the electronic,shadow-indicating type which operates from the output of said balanceddetector and which indicates a correct adjustment of tuning of saidreceiver by a definite shape of shadow, and carrier-operated meansconnected to said indicator means in such a manner that said definiteshape of shadow appears only when a signal is present in the output ofthe selective amplifier.

5. In a receiver for frequency-modulated waves including a selectiveamplifier and a balanced detector whose rectified output issubstantially proportional, over the normal operating range, to thedeparture in frequency of a selectively amplified incoming signal fromits mean value, a tuning indicator means of the electronic,shadowindicating type which operates from the output of said balanceddetector and which indicates a correct adjustment of tuning of saidreceiver by a definite shape of shadow, and carrier-operated meansconnected to said indicator means to control the latter whereby saiddefinite shape of shadow appears only when a signal is present in theoutput of the selective amplifier, said carrieroperated means consistingof an electron discharge tube whose plate current flow is interrupted bya direct voltage derived from the magnitude of said signal independentlyof its frequency.

6. In a receiver for frequency-modulated waves including aselective'amplifier and a balanced detector whose rectified output issubstantially proportionaLover the normal operating range, to thedeparture in frequency of a selectively amplified incoming signal fromits mean value, a tuning indicator means of the electronic,shadow-indicating type which operates from the output of said balanceddetector and which indicates a correct adjustment of 'tuning of saidreceiver by a definite shape of shadow, and additional means foroperating said tuning indicator by a direct voltage derived from theamplitude of said signal independently of its frequency variations.

7. In combination with a frequency modulated carrier wave detectorhaving an input circuit upon which are impressed frequency modulatedcarrier waves, said detector having an output circuit across which isdeveloped direct current voltage whose polarity and magnitude dependupon the frequency departure of the center frequency of the appliedmodulated waves from an assigned center frequency value of said detectorinput circuit, an electronic shadow indicator tube provided with ashadow control electrode, means for varying the potential of said shadowcontrol electrode in response to said detector direct current voltageoutput whereby the shadow angle of the indicator tube indicates the saidfrequency departure, tube means for controlling the potential of saidshadow control electrode in the absence of frequency modulated waves atsaid input circuit-for maintaining the shadow of said indicator tube atmaximum width, and additional means, responsive to carrier amplitudeincrease,

for rendering said tube means ineffective.

8. In a frequency modulation receiver provided with-adetection networkof the type comprising a pair of rectifiers having a common inputcircuit resonated to a predetermined center frequency, an output circuitacross which is developed a direct current voltage whose polarity andmagnitude are a function of the frequency departure of applied modulatedwaves from said center frequency, an amplifier for amplifying saiddirect current coltage, an indicator tube provided with an emissionelectrode, a fluorescent target, and

an electron ray control electrode, means for aptaining the shadow onsaid target at a relatively wide angle.

9. In combination with a variable frequency wave detector having aninput circuit upon which are impressed variable frequency waves, saiddetector having an output circuit across which is developed arectifiedvoltage whose polarity and magnitude depend upon the frequencydeparture of the center frequency of the applied waves from an assignedcenter frequency value of said detector input circuit, an indicator tubeof the fluorescent target type provided with a shadow control electrode,means for varying the potential of said shadow control electrode inresponse to said detector voltage output whereby the shadow angle of theindicator tube indicates the said frequency departure, a normallyoperative tube for controlling the potential of said shadow controlelectrode in the absence of waves at said input circuit for maintainingthe shadow of said indicator tube at maximum width, and additional meansresponsive to carrier amplitude increase for rendering said last tubeineffective.

10. In combination with a variable frequency wave detector having aninput circuit upon which are ipressed variable frequency waves, saiddetector having an output circuit across which is developed a rectifiedvoltage whose polarity and magnitude depend upon the frequency departureof the center frequency of the applied waves from an assigned centerfrequency value of said detector input circuit, an indicator tube of thefluorescent target type provided with a shadow control electrode, meansfor varying the potential of said shadow control electrode in responseto said detector voltage output whereby the shadow angle of the indictortube indicates the said frequency departure, a normally operative tubefor controlling the potential of said shadow control electrode in theabsence of waves at said input circuit for maintaining the shadow ofsaid indicator tube at maximum width, and additional means responsive tocarrier amplitude increase.

for rendering said last tube ineffective, said additional meansconsisting of a direct current voltage connection between said normallyoperative tube and a point on said detector output circuit whose voltagevaries with carrier amplitude change.

11.- In a frequency modulation receiver provided with a detectionnetwork of the type comprising a pair of rectifiers having a commoninput circuit resonated to a predetermined center frequency, an outputcircuit across which is developed a direct current voltage whosepolarity and magnitude are a function of the frequency departure ofapplied modulated waves from said center frequency, an amplifier foramplifying said direct current voltage, an indicator tube provided withan emission electrode, a fluorescent target, and an electron ray controlelectrode,

means for applying the direct current voltage output of said amplifierto said ray control electrode thereby to control the width of angle ofthe shadow produced on the target, and means including a tube, andresponsive to a decrease of modulated carrier amplitude below a desiredvalue, for maintaining the shadow on said target at a relatively wideangle, and the electrodes of said amplifier being located in tubeenvelope of said last mentioned tube,

12. In a receiver for frequency-modulated waves including a selectiveamplifier and a balanced detector whose rectified output issubstantially proportional, over the normal operating 2,286,412 range,to the departure in frequency of a selectude of said signalindependently of its frequency variations, said additional meansincluding an electron discharge device having an output connection tosaid indicator means, and means for applying said derived direct currentvoltage to said device from a. point of the receiver preceding saiddetector.

EDWARD W. HEROLD.

